Conventionally, so-called delayed fracture is known, i.e., sudden brittle fracture of a steel product under static stress after the lapse of a certain time. Such delayed fracture is believed to be caused, as one of factors, by atomic hydrogen (H) intervening in interstitial iron atom or atomic hydrogen trapped in lattice defects rather than molecular hydrogen (H2) trapped in voids within steel or interfaces between steel matrix and non-metallic inclusion.
The atomic diffusive hydrogen is (1) hydrogen within molten steel which remains in final steel products or steel parts; (2) hydrogen penetrating into a steel product or steel parts due to corrosion under the atmospheric environment or during pickling or electroplating process; and (3) hydrogen penetrating into a steel product or steel parts from the atmosphere during carburizing, nitridation, welding; and the like. The delayed fracture is believed to occur when a hydrogen concentration within a steel product exceeds a critical hydrogen concentration of fracture.
In steel parts, screws such as wood screws, tapping screws and the like are formed using a low carbon aluminum killed steel wire such as SWRCH18A through cold forging. These screws are subjected to a heat treatment such as carburizing and quenching after formation for improving screw performance of a screw tip when a screw is driven in, and increasing a torsional torque before the screw surfaces are plated by electrogalvanizing having the plating thickness of 5 to 20 μm for improving the corrosion resistance. Once hydrogen generated by the plating penetrates into a steel product, the hydrogen diffuses and readily migrates into screws, while a plated coating on the surface makes it difficult to remove the hydrogen from the surface, resulting in a higher hydrogen concentration within the screws. For this reason, as a certain time has elapsed after a screw was fastened with a predetermined torque, the screw is susceptible to a delayed fracture, so-called head-off of the screw.
In addition, screws hardened by the carburizing and quenching are formed with high carbon tempered martensite, in a surface layer, with approximately 0.8% of carbon. Since this tempered martensite is precipitated as grain boundary carbide at prior austenite grain boundaries, and agglomerated hydrogen further reduces a bonding strength of grain boundary, resulting in higher susceptibility to hydrogen embrittlement and hence higher susceptibility to delay fracture.
Further, steel parts manufactured from carbon steel or low-alloy steel, such as bolts, pins, washers, shafts, plates and the like are also increased in susceptibility to hydrogen embrittlement due to hydrogen in molten steel, or hydrogen penetrating into them during pickling to remove a scale on the surface of a steel product.
Therefore, conventionally, steel parts such as plated screws and the like have been baked, for example, at temperatures of approximately 200° C. for several hours in a furnace to reduce hydrogen concentrations therein.
However, the baking in a furnace may require to maintain heating for approximately four hours depending on the amount of hydrogen concentration or the size of part. Therefore, if the baking is conducted in a batch-type furnace, the productivity will be degraded, the total stock will be increased, and enhancement of facilities will be required. Also, when the baking is conducted using a continuous-type furnace, a long furnace having a long conveyer is required, resulting in a higher manufacturing cost.